This invention relates to circuit pressure control systems for hydrostatic power transmissions, and more particularly, it is concerned with a circuit pressure control system for a hydrostatic power transmission of a hydraulically operated machine, such as a bulldozer, hydraulic shovel, hydraulic crane, etc.
In one type of control system known in the art for a hydrostatic power transmission of a hydraulically operated machine, such as a bulldozer, hydraulic shovel, hydraulic crane, etc., a variable-displacement hydraulic pump driven by a prime mover is connected to a hydraulic actuator for actuating a load in closed or semi-closed circuit and the speed of the hydraulic actuator is controlled by varying the displacement of the hydraulic pump. A swash-plate pump of the reversible tilt type is used, for example, as a variable-displacement hydraulic pump. A displacement adjusting mechanism connected to a hydraulic pressure source via a servo valve is used as means for varying the hydraulic pump displacement. When the servo valve is supplied with an operating current commensurate in value with the deviation of a hydraulic pump swash-plate tilt (hydraulic pump displacement) signal Y from an operating lever manipulated variable signal X.sub.L, it operates to bring the displacement adjusting mechanism into communication with the hydraulic pressure source to thereby control the hydraulic pump swash-plate tilt to render the swash-plate tilt Y equal to the operating lever manipulated variable signal X.sub.L.
In a closed circuit hydrostatic power transmission, a hydraulic motor is usually employed as a hydraulic actuator, and an auxiliary pump for merely supplying the hydraulic fluid to compensate for leaks from the main circuit is provided.
In a semi-closed circuit hydrostatic power transmission, a hydraulic cylinder is usually employed as a hydraulic actuator, and when the hydraulic cylinder is actuated the difference between the supply and discharge of the working fluid due to the difference in volume between the supply side and the discharge side of the cylinder is released through a flushing valve from the main circuit.
In such hydrostatic power transmission, abrupt actuation of the operating lever would cause a sudden increase in the delivery of the hydraulic pump, and the circuit pressure would become inordinately high due to the inertia of the load driven by the hydraulic actuator. This tendency would be marked when the inertia of the load is high. To avoid this phenomenon, conduits of the main circuit have mounted thereacross a crossover relief valve for releasing the difference between the delivery by the hydraulic pump and the suction by the hydraulic actuator. The working fluid thus released represents a loss of energy.
In order to avoid the loss of energy referred to hereinabove, proposals have been made to use circuit pressure control means. One of such proposals involves a circuit pressure control system described in "MACHINE DESIGN", pages 114-116, issued on Oct. 7, 1976. This system includes a three-way change-over valve mounted between the hydraulic fluid inlet of the servo valve connected to the hydraulic pump displacement adjusting mechanism and the hydraulic pressure source. The servo valve has a spring mounted in one pilot section thereof, and the circuit pressure of the hydrostatic power transmission is caused to act on the other pilot section thereof, so that when the hydraulic actuator is accelerated the three-way change-over valve is actuated to decrease the volume of hydraulic fluid supplied through the servo valve to the displacement adjusting mechanism as the circuit pressure rises above the value set by the spring, to thereby decrease the rate of increase of the delivery by the hydraulic pump and avoid the circuit pressure rising to an inordinately higher value than the value set by the spring. Thus it is possible to avoid a loss of energy occurring when the excess fluid in the main circuit is released through the crossover relief valve.
As described hereinabove, the aforesaid type of circuit pressure control system is capable of performing the desired pressure control function to avoid an inordinate rise in circuit pressure, when the hydraulic actuator is accelerated. However, a rise in circuit pressure occurs not only when the hydraulic actuator is accelerated but also in other operating conditions in which the hydraulic actuator functions as a hydraulic pump. In such operating conditions, it is desired that the energy produced by the operation of the hydraulic actuator as a hydraulic pump be recovered by the prime mover through the hydraulic pump. The circuit pressure control system of the type described hereinabove has been unable to effect control as desired in such operating conditions, with a result that the recovery of the energy has not been effected as desired.
More specifically, when the hydraulic actuator or motor is accelerated in the positive direction, for example, a circuit pressure on the discharge side of the hydraulic pump would rise. If, thereafter, the operating lever is restored to obtain deceleration, a circuit pressure on the suction side of the hydraulic pump would rise since the hydraulic actuator functions as a pump when the hydraulic actuator is decelerated. It is generally desired that the kinetic energy of the hydraulic motor and the load be recovered by the prime mover in the form of power recovery through the hydraulic pump. To realize the power recovery, the rate of a reduction in the pump displacement or swash-plate tilt is required to be restricted to avoid a sudden reduction in the delivery by the hydraulic pump. In the aforesaid circuit pressure control system, however, the three-way changeover valve is actuated with a rise in circuit pressure, and the pressure applied to the hydraulic fluid supply port of the servo valve communicating with the hydraulic pressure source through the three-way change-over valve is decreased. Thus no hydraulic fluid is supplied to the displacement adjusting mechanism and the pump swash-plate is moved toward a neutral position by the action of swash-plate neutral restoration springs of the displacement adjusting mechanism. As a result, it is impossible to effectively control the circuit pressure and to achieve power recovery.
Not only when the hydraulic actuator is actuated but also in case an external force is exerted on the output shaft of the hydraulic actuator to forcedly actuate same when the hydraulic actuator is operated at a constant speed or when it is accelerated, the hydraulic actuator would function as a pump and the circuit pressure would show an inordinate rise in the event that the external force is excessively high in magnitude. In such a case, since the aforesaid circuit pressure control system has no function of increasing the hydraulic pump swash-plate tilt to cope with a rise in its circuit pressure, it is impossible for the system to increase the suction by the hydraulic pump, thereby making it impossible to achieve effective power recovery.